
This is PG-13 -- don't read if a story about a virus might make you queasy
The flu story...it's been awhile since I studied this, so this is dredged up from my addled memory.
Once a person catches the flu (and survives it), that person can't catch that flu any more. The flu must first mutate, and one of the common ways it mutates involves three species.
Humans catch the flu. Ducks catch the flu. Ducks cannot catch human flu, and humans cannot catch duck flu -- the flu virii cannot jump species between humans and ducks.
There are few diseases we can catch from cows. The spongeform prion may be contageous, as well as TB, but if the cow is reasonably healthy, we can eat beef rare.
Enter the pig. Pigs and humans can swap virii fairly easily, which is why we have to cook the hell out of pork. When it comes to species jumping virii, pigs are truly 'unclean'. It turns out that pigs and ducks can swap flu as well.
So, zoom in to a tiny farm in Asia. There's a farmer, sick with the flu, feeding a few pigs. The pigs trot over and drink some water from a tiny pond. Some ducks fly through, spend the night in the pond, then fly on in the morning. Unfortunately, one of these ducks had the duck flu. One pig gets sick from the farmer's flu, then the duck flu. This poor pig is hit by two flu virii simultaneously. The virii sometimes attack the same lung cells and mutate within the pig. Three days later the farmer, just getting over the flu from a few days before, gets hit again. The original duck may be miles away, perhaps dead, but other ducks visit the pond and spread this new strain among the duck population.
The farmer spreads the flu sometimes just by being in the same room as someone. Sometimes it's by touch. Sometimes it's by handling porous objects, such as money or paper or fabric. After a while, the flu hits the airport.
For the USA, ground zero for new flu strains is Orlando, Florida. There are outbreaks in all the major cities as well -- New York, Chicago, DC, LA -- but the most common entry vector occurs in the Orlando airport. It's tourists, see... Europeans, Africans, Asians come to Disney World. Americans from all over the States come to Disney World. They share the same recycled air in the planes. They jostle against each other in the airport and that little tram that takes you from the gates to the terminal. They stand in line together at the car rental place. They handle a lot of money.
They have their vacation and go home, perhaps with a little cough or a buzzy head that they figure comes from overexerting themselves. They go back to Paris or Des Moines or Capetown or Kyoto....